PythonBuffering.java

import static java.util.Arrays.asList;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.file.Files;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

public class PythonBuffering {

	// run python in buffering mode (pyhon default) or unbuffered using "-u"
	static boolean runWithMinusU = false;
	// call sys.stdout.flush() in py code
	static boolean forceFlush = false;

	public static void main(String[] args) throws IOException {
		BufferedReader reader = new BufferedReader(new InputStreamReader(createProcess().getInputStream()));
		String line;
		while ((line = reader.readLine()) != null) {
			System.out.println(new Date() + " " + line);
		}
	}

	private static Process createProcess() throws IOException, FileNotFoundException {
		List<String> pyArgs = new ArrayList<>(asList("python", createPyFile().getAbsolutePath()));
		if (runWithMinusU) {
			pyArgs.add(1, "-u");
		}
		return new ProcessBuilder().command(pyArgs).redirectErrorStream(true).start();
	}

	private static File createPyFile() throws IOException, FileNotFoundException {
		File pyCode = File.createTempFile("buffering-showcase-", ".py");
		pyCode.deleteOnExit();
		Files.write(pyCode.toPath(), pyCode());
		return pyCode;
	}

	private static List<String> pyCode() {
		List<String> lines = new ArrayList<>();
		lines.add("#!/usr/bin/python");
		lines.add("");
		lines.add("import sys");
		lines.add("import time");
		lines.add("");
		lines.add("x = 1");
		lines.add("while True:");
		lines.add("    print(x)");
		if (forceFlush) {
			lines.add("    sys.stdout.flush()");
		}
		lines.add("    time.sleep(0.01)");
		lines.add("    x += 1");
		lines.add("");
		return lines;
	}

}